为什么“traceId”返回消息字符串本身？ [英] Why does `traceId` return the message string itself?

问题描述

我发现这很奇怪， traceId 返回消息字符串本身。为什么有人想这样做？

有一个常见的情况是， traceId 会派上用场吗？ p>

解决方案

编辑：

向David提醒，David提醒了我的 traceId'正好是 traceShowId 。 p>

因此， traecId 只是 String - > String 单态版本的多态 traceShowId ::显示 - > a - >

旧答案

解释 traceId 的一种方法是它是一个 String 专门版本的更多一般 traceId'不在stdlib中：

  traceId':: Show a => a  - > a 
 traceId'x = trace（show x）x 
  

所以

  traceId = traceId':: String  - > String 
  

示例：

  my = id; func = id; chain = id; value = 3 
 
 main = do 
 print $ length $ traceIdchain is called！ 
 print $ length $ traceId'[1,2,3] 
  

但由于某种原因 traceId 的一般版本，我称之为 traceId'，不在stdlib中。

I found it really weird that traceId returns the message string itself. Why would anyone want to do that?

Is there a common scenario where traceId would come in handy?

解决方案

EDIT:

Credits to David who drew attention on the fact that my traceId' is exactly traceShowId.

Hence, traecId is simply the String -> String monomorphic version of the polymorphic traceShowId :: Show a -> a -> a, which also strips the printed string from quotes.

Old answer:

One way to interpret traceId is that it's a String-specialized version of a more general traceId' which is not in the stdlib:

traceId' :: Show a => a -> a
traceId' x = trace (show x) x

so that

traceId = traceId' :: String -> String

Example:

my = id; func = id; chain = id; value = 3

main = do
  print $ length $ traceId "chain is called!"
  print $ length $ traceId' [1,2,3]

but for some reason the general version of traceId, which I've called traceId', is not in the stdlib.

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